System and method for channel estimation in a delay diversity wireless communication system

ABSTRACT

A method of controlling downlink transmissions to a subscriber station capable of communicating with a base station of an orthogonal frequency division multiplexing (OFDM) network. The method comprises the steps of: receiving a first pilot signal from a first base station antenna; receiving a second pilot signal from a second base station antenna; and estimating the channel between the base station and subscriber station based on the received first and second pilot signals. The method also comprises determining a set of OFDM symbol processing parameters based on the step of estimating the channel and transmitting the OFDM symbol processing parameters to the base station. The base station uses the OFDM symbol processing parameters to control the relative gains and the relative delays of OFDM symbols transmitted from the first and second antennas.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/390,125, filed Mar. 27, 2006 now U.S. Pat. No. 7,953,039, entitled“SYSTEM AND METHOD FOR CHANNEL ESTIMATION IN A DELAY DIVERSITY WIRELESSCOMMUNICATION SYSTEM,” which claims priority to U.S. Provisional PatentApplication No. 60/673,574 filed Apr. 21, 2005, U.S. Provisional PatentApplication No. 60/673,674 filed Apr. 21, 2005 and U.S. ProvisionalPatent Application No. 60/679,025 filed May 9, 2005. U.S. patentapplication Ser. No. 11/390,125 is assigned to the assignee of thepresent application and is incorporated by reference into thisdisclosure as if fully set forth herein. This disclosure hereby claimspriority under 35 U.S.C. §120 to U.S. patent application Ser. No.11/390,125.

CROSS-REFERENCE TO RELATED APPLICATIONS AND CLAIM OF PRIORITY

The present application is related to Prov. Pat. Nos. 60/673,574 and60/673,674, both entitled “Diversity Transmission in an OFDM WirelessCommunication System” and filed Apr. 21, 2005, and to Prov. Pat. No.60/679,026, entitled “Channel Estimation in a Delay Diversity WirelessCommunication System,” and filed May 9, 2005. Prov. Pat. Nos.60/673,574, 60/673,674, and 60/679,026 are assigned to the assignee ofthis application. The subject matter disclosed in Prov. Pat. Nos.60/673,574, 60/673,674, and 60/679,026 is hereby incorporated byreference. This application claims priority under 35 U.S.C. §119(e) toProv. Pat. Nos. 60/673,574, 60/673,674, and 60/679,026.

The present application is related to U.S. patent application Ser. No.11/327,799, entitled “Method And System For Introducing FrequencySelectivity Into Transmissions In An Orthogonal Frequency DivisionMultiplexing Network”, filed Jan. 6, 2006. Application Ser. No.11/327,799 is assigned to the assignee of the present application. Thesubject matter disclosed in application Ser. No. 11/327,799 is herebyincorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present disclosure relates generally to wireless communications and,more specifically, to an apparatus and method for performing channelestimation in an orthogonal frequency division multiplexing (OFDM)network or an orthogonal frequency division multiple access (OFDMA)network.

BACKGROUND OF THE INVENTION

Conventional orthogonal frequency division multiplexing (OFDM) networksand orthogonal frequency division multiple access (OFDMA) network areable to improve the reliability of the channel by spreading and/orcoding data traffic and control signals over multiple subcarriers (i.e.,tones). However, if the channel is flat, frequency diversity cannot beachieved. In order to overcome this, it is possible to introduceartificial frequency diversity into the transmitted signal. A techniquefor artificially introducing frequency diversity into an OFDMenvironment was disclosed in U.S. patent application Ser. No.11/327,799, filed on Jan. 6, 2006 and incorporated by reference above.In the device disclosed in Ser. No. 11/327,799, multiple copies of thesame OFDM symbol are delayed by different delay values, then amplifiedby the same or different gain values, and then transmitted fromdifferent antennas. This artificially introduces frequency-selectivefading in the ODFM channel, thereby allowing frequency selectivity to beexploited using frequency-domain scheduling for low-to-medium speedmobile devices or frequency diversity for higher speed mobile devices.

However, when selecting the symbol processing parameters (i.e., delayvalues and the gain values) applied to the OFDM symbols, it is importantto take into consideration the user channel type and the mobile speed.To accomplish this, channel estimation is performed and the symbolprocessing parameters are determined based on the channel estimates andmobile speed. Therefore, there is a need for improved apparatuses andmethods for performing channel estimation in an OFDM wireless networkthat artificially introduces frequency diversity by delaying andamplifying multiple copies of the same OFDM symbol and then transmittingthe delayed and amplified OFDM symbols from different transmit antennas.

SUMMARY OF THE INVENTION

A method of controlling downlink transmissions to a subscriber stationis provided for use in a subscriber station capable of communicatingwith a base station of an orthogonal frequency division multiplexing(OFDM) network. The method comprises the steps of: receiving a firstpilot signal from a first antenna of the base station; receiving asecond pilot signal from a second antenna of the base station;estimating the channel between the base station and subscriber stationbased on the received first and second pilot signals; determining a setof OFDM symbol processing parameters based on the step of estimating thechannel, wherein the OFDM symbol processing parameters are usable by thebase station to control the relative gains and the relative delays ofOFDM symbols transmitted from the first and second antennas; andtransmitting the OFDM symbol processing parameter set to the basestation.

According to another embodiment of the present disclosure, a subscriberstation capable of communicating with a base station of an orthogonalfrequency division multiplexing (OFDM) network is provided. Thesubscriber station comprises: receive path circuitry capable ofreceiving a first pilot signal from a first antenna of the base stationand receiving a second pilot signal from a second antenna of the basestation; and channel estimating circuitry capable of estimating thechannel between the base station and subscriber station based on thereceived first and second pilot signals and capable of determining a setof OFDM symbol processing parameters based on a channel qualityestimate. The OFDM symbol processing parameters are usable by the basestation to control the relative gains and the relative delays of OFDMsymbols transmitted from the first and second antennas and wherein thesubscriber station is capable of transmitting the OFDM symbol processingparameters to the base station.

According to yet another embodiment of the present disclosure, a basestation is provided for use in an orthogonal frequency divisionmultiplexing (OFDM) network. The base station comprises: 1) receive pathcircuitry capable of receiving an uplink signal from a subscriberstation, estimating the channel between the base station and subscriberstation based on the received uplink signal, and determining a set ofOFDM symbol processing parameters based on a channel quality estimate;and 2) transmit path circuitry capable of using the OFDM symbolprocessing parameters to control the relative gains and the relativedelays of processed OFDM symbols transmitted from a first antenna and asecond antenna of the base station. The base station is capable oftransmitting the OFDM symbol processing parameters to the subscriberstation. The OFDM symbol processing parameters are based on themultipath characteristics and the frequency selectivity characteristicsof the channel.

Before undertaking the DETAILED DESCRIPTION OF THE INVENTION below, itmay be advantageous to set forth definitions of certain words andphrases used throughout this patent document: the terms “include” and“comprise,” as well as derivatives thereof, mean inclusion withoutlimitation; the term “or,” is inclusive, meaning and/or; the term “each”means every one of at least a subset of the identified items; thephrases “associated with” and “associated therewith,” as well asderivatives thereof, may mean to include, be included within,interconnect with, contain, be contained within, connect to or with,couple to or with, be communicable with, cooperate with, interleave,juxtapose, be proximate to, be bound to or with, have, have a propertyof, or the like; and the term “controller” means any device, system orpart thereof that controls at least one operation, such a device may beimplemented in hardware, firmware or software, or some combination of atleast two of the same. It should be noted that the functionalityassociated with any particular controller may be centralized ordistributed, whether locally or remotely. Definitions for certain wordsand phrases are provided throughout this patent document, those ofordinary skill in the art should understand that in many, if not mostinstances, such definitions apply to prior, as well as future uses ofsuch defined words and phrases.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure and itsadvantages, reference is now made to the following description taken inconjunction with the accompanying drawings, in which like referencenumerals represent like parts:

FIG. 1 illustrates an exemplary orthogonal frequency divisionmultiplexing (OFDM) wireless network that is capable of performingchannel estimation according to the principles of the presentdisclosure;

FIG. 2A is a high-level diagram of the orthogonal frequency divisionmultiplexing (OFDM) transmit path in a base station according to oneembodiment of the disclosure;

FIG. 2B is a high-level diagram of the orthogonal frequency divisionmultiplexing (OFDM) receive path in a subscriber station according toone embodiment of the disclosure;

FIG. 3 illustrates the OFDM symbol processing block in the base stationin greater detail according to an exemplary embodiment of the presentdisclosure;

FIG. 4A illustrates data traffic transmitted in the downlink from a basestation to a subscriber station according to an exemplary embodiment ofthe present disclosure;

FIG. 4B is a flow diagram illustrating the determination of the userchannel type based on the measurements on the preamble according to anexemplary embodiment of the disclosure;

FIG. 5 is a message flow diagram illustrating the transmission of OFDMsymbols from a base station to a subscriber station according to theprinciples of the disclosure;

FIG. 6 is a flow diagram illustrating the processing of pilot signalsand OFDM data symbols according to an exemplary embodiment of thepresent disclosure;

FIG. 7 is a message flow diagram illustrating the transmission of OFDMsymbols from a base station to a subscriber station according to anotherembodiment of the disclosure; and

FIG. 8 is a message flow diagram illustrating the transmission of OFDMsymbols from a base station to a subscriber station according to anotherembodiment of the disclosure.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1 through 8, discussed below, and the various embodiments used todescribe the principles of the present disclosure in this patentdocument are by way of illustration only and should not be construed inany way to limit the scope of the disclosure. Those skilled in the artwill understand that the principles of the present disclosure may beimplemented in any suitably arranged wireless network.

The present disclosure is directed to apparatuses and algorithms forchannel estimation and channel quality estimation for demodulation anddata rate selection in an orthogonal frequency division multiplexing(OFDM) wireless network that uses delayed diversity. Such a delayeddiversity wireless network was disclosed previously U.S. patentapplication Ser. No. 11/327,799, incorporated by reference above. Thepresent disclosure uses a number of factors, including user channel typeand mobile speed, to select OFDM symbol processing parameters (i.e.,delays D1, D2, . . . , DP and gains g₀, g₁, . . . , g_(p)) for OFDMsymbols transmitted from up to P antennas (i.e., ANT1 to ANTP).Therefore, different OFDM symbol processing parameters may be used totransmit to different mobile devices that are scheduled simultaneously,depending upon their channel types.

It is noted that the scope of the present disclosure is not limited toorthogonal frequency division multiplexing (OFDM) wireless networks. Thepresent disclosure is also applicable to orthogonal frequency divisionmultiple access (OFDMA) wireless networks. However, for simplicity andbrevity, the embodiments described below are directed to OFDM wirelessnetworks, except where otherwise noted or where the context indicatesotherwise.

For relatively low-speed mobile devices, it is usually possible to trackchanges in the channel, thereby allowing channel sensitive scheduling toimprove performance. Thus, the OFDM symbol processing parameters may beselected in such a way that relatively large coherence bandwidthresults. That is, a relatively larger number of subcarriers experiencesimilar fading. This goal may be achieved by keeping the delays for OFDMsymbols from different antennas relatively small. A mobile device maythen be scheduled on a subband consisting of contiguous subcarriers.

For relatively high-speed mobile devices, channel quality variationscannot be tracked accurately, so that frequency-diversity may behelpful. Thus, the OFDM symbol processing parameters are selected insuch a way that relatively small coherence bandwidth results. That is,potentially independent fading may occur from subcarrier to subcarrier.This goal may be achieved by having relatively large delays for OFDMsymbols transmitted from different antennas.

The symbol processing parameters may also be selected based on thedegree of frequency-selectivity already present in the channel. Forexample, if a channel already has a lot of multipath effects and is,therefore, frequency selective, there may be little or no need foradditional frequency selectivity. The OFDM symbol processing parametersmay be selected on a user-by-user basis because different mobile devicesexperience different channel types.

FIG. 1 illustrates exemplary orthogonal frequency division multiplexing(OFDM) wireless network 100, which is capable of performing channelestimation according to the principles of the present disclosure. In theillustrated embodiment, wireless network 100 includes base station (BS)101, base station (BS) 102, base station (BS) 103, and other similarbase stations (not shown). Base station 101 is in communication withbase station 102 and base station 103. Base station 101 is also incommunication with Internet 130 or a similar IP-based network (notshown).

Base station 102 provides wireless broadband access (via base station101) to Internet 130 to a first plurality of subscriber stations withincoverage area 120 of base station 102. The first plurality of subscriberstations includes subscriber station 111, which may be located in asmall business (SD), subscriber station 112, which may be located in anenterprise (E), subscriber station 113, which may be located in a WiFihotspot (HS), subscriber station 114, which may be located in a firstresidence (R), subscriber station 115, which may be located in a secondresidence (R), and subscriber station 116, which may be a mobile device(M), such as a cell phone, a wireless laptop, a wireless PDA, or thelike.

Base station 103 provides wireless broadband access (via base station101) to Internet 130 to a second plurality of subscriber stations withincoverage area 125 of base station 103. The second plurality ofsubscriber stations includes subscriber station 115 and subscriberstation 116. In an exemplary embodiment, base stations 101-103 maycommunicate with each other and with subscriber stations 111-116 usingOFDM or OFDMA techniques.

Base station 101 may be in communication with either a greater number ora lesser number of base stations. Furthermore, while only six subscriberstations are depicted in FIG. 1, it is understood that wireless network100 may provide wireless broadband access to additional subscriberstations. It is noted that subscriber station 115 and subscriber station116 are located on the edges of both coverage area 120 and coverage area125. Subscriber station 115 and subscriber station 116 each communicatewith both base station 102 and base station 103 and may be said to beoperating in handoff mode, as known to those of skill in the art.

Subscriber stations 111-116 may access voice, data, video, videoconferencing, and/or other broadband services via Internet 130. In anexemplary embodiment, one or more of subscriber stations 111-116 may beassociated with an access point (AP) of a WiFi WLAN. Subscriber station116 may be any of a number of mobile devices, including awireless-enabled laptop computer, personal data assistant, notebook,handheld device, or other wireless-enabled device. Subscriber stations114 and 115 may be, for example, a wireless-enabled personal computer(PC), a laptop computer, a gateway, or another device.

FIG. 2A is a high-level diagram of the transmit path in orthogonalfrequency division multiplexing (OFDM) transmitter 200 according to anexemplary embodiment of the disclosure. FIG. 2D is a high-level diagramof the receive path in orthogonal frequency division multiplexing (OFDM)receiver 260 according to an exemplary embodiment of the disclosure.OFDM transmitter 200 comprises quadrature amplitude modulation (QAM)modulator 205, serial-to-parallel (S-to-P) block 210, Inverse FastFourier Transform (IFFT) block 215, parallel-to-serial (P-to-S) block220, add cyclic prefix block 225, and OFDM symbol processing block 230.OFDM receiver 250 comprises remove cyclic prefix block 260,serial-to-parallel (S-to-P) block 265, Fast Fourier Transform (FFT)block 270, parallel-to-serial (P-to-S) block 275, quadrature amplitudemodulation (QAM) demodulator 280, and channel estimation block 285.

At least some of the components in FIGS. 2A and 2B may be implemented insoftware while other components may be implemented by configurablehardware or a mixture of software and configurable hardware. Inparticular, it is noted that the FFT blocks and the IFFT blocksdescribed in FIGS. 2A and 2B may be implemented as configurable softwarealgorithms, where the values of FFT and IFFT sizes may be modifiedaccording to the implementation.

QAM modulator 205 receives a stream of input data and modulates theinput bits (or symbols) to produce a sequence of frequency-domainmodulation symbols. Serial-to-parallel block 210 converts (i.e.,de-multiplexes) the serial QAM symbols to parallel data to produce Mparallel symbol streams where M is the IFFT/FFT size used in OFDMtransmitter 200 and OFDM receiver 250. IFFT block 215 then performs anIFFT operation on the M parallel symbol streams to produce time-domainoutput signals. Parallel-to-serial block 220 converts (i.e.,multiplexes) the parallel time-domain output symbols from IFFT block 215to produce a serial time-domain signal.

Add cyclic prefix block 225 then inserts a cyclic prefix to each OFDMsymbol in the time-domain signal. As is well known, the cyclic prefix isgenerated by copying the last G samples of an N sample OFDM symbol andappending the copied G samples to the front of the OFDM symbol. Finally,OFDM symbol processing block 230 processes the incoming OFDM symbols asdescribed in FIG. 3 and as described in U.S. patent application Ser. No.11/327,799. The process OFDM samples at the output of OFDM symbolprocessing block 230 are then sent to up-conversion circuitry (notshown) prior to being transmitted from multiples transmit antennas.

The transmitted RF signal arrives at OFDM receiver 250 after passingthrough the wireless channel and reverse operations to those in OFDMtransmitter 200 are performed. Remove cyclic prefix block 260 removesthe cyclic prefix to produce the serial time-domain baseband signal.Serial-to-parallel block 265 converts the time-domain baseband signal toparallel time domain signals. FFT block 270 then performs an FFTalgorithm to produce M parallel frequency-domain signals.Parallel-to-serial block 275 converts the parallel frequency-domainsignals to a sequence of QAM data symbols. QAM demodulator 280 thendemodulates the QAM symbols to recover the original input data stream.Channel estimation block 285 also receives the QAM data symbols fromparallel-to-serial block 275 and performs channels estimation. As willbe described below in greater detail, the channel estimation values areused to determine a parameter set of gain values and delay values thatare used in OFDM symbol processing block 230 in OFDM transmitter 200 andare used by QAM demodulator 280 to demodulate the QAM data symbols.

The exemplary transmit path of OFDM transmitter 200 may berepresentative of the transmit paths of any one of base stations 101-103or any one of subscriber stations 111-116. Similarly, the exemplaryreceive path of OFDM receiver 250 may be representative of the transmitpaths of any one of base stations 101-103 or any one of subscriberstations 111-116. However, since multiple antenna configurations aremore common in base stations than in subscriber stations or other mobiledevices, for the sake of simplicity and clarity, the descriptions thatfollow will be directed toward transactions between a base station(e.g., BS 102) that implements a transmit path similar to OFDMtransmitter 200 and a subscriber station (e.g., SS 116) that implementsa receive path similar to OFDM receiver 250. However, such an exemplaryembodiment should not be construed to limit the scope of the presentdisclosure. It will be appreciated by those skilled in the art that incases where multiple antennas are implemented in a subscriber station,the transmit path and the receiver path of both the base station and thesubscriber station may be implemented as in shown in FIGS. 2A and 2B.

FIG. 3 illustrates OFDM symbol processing block 230 in greater detailaccording to an exemplary embodiment of the present disclosure. OFDMsymbol processing block 230 comprises P delay elements, includingexemplary delay elements 311 and 312, P+1 amplifiers, includingexemplary amplifiers 321, 322 and 323, and P+1 transmit antennas,including exemplary antennas 331, 332 and 333. Delay elements 311 and312 are arbitrarily labeled “D1” and “DP”, respectively. OFDM symbolprocessing block 230 receives incoming ODFM symbols and forwards P+1copies of each OFDM symbol to the P+1 transmit antennas. Each OFDMsymbol comprises N+G samples, where N is the number of samples in theoriginal data symbol and G is the number of samples in the cyclic prefixappended to the original symbol.

A first copy of each OFDM symbol is applied directly to the input ofamplifier 321, amplified by a gain value, g0, and sent to antenna 331. Asecond copy of each OFDM symbol is delayed by delay element 311, appliedto the input of amplifier 322, amplified by a gain value, g1, and sentto antenna 332. Other copies of each OFDM symbol are similarly delayedand amplified according to the number of antennas. By way of example,the P+1 copy of each OFDM symbol is delayed by delay element 312,applied to the input of amplifier 323, amplified by a gain value, gP,and sent to antenna 333. The gain values and the delay values aredetermined by the values in an OFDM symbol processing parameter set, asdescribed hereafter and in U.S. patent application Ser. No. 11/327,799.The result is that multiple copies of each OFDM are transmitted, whereineach copy of an OFDM symbol is amplified by a selected amount anddelayed by a selected amount relative to other OFDM symbol copies. U.S.patent application Ser. No. 11/327,799, incorporated by reference above,describes a number of architectures for OFDM symbol processing block 230that achieve such a result. In an advantageous embodiment, the delaysintroduced by OFDM symbol processing block 230 are cyclic delays, asdisclosed in U.S. patent application Ser. No. 11/327,799.

FIG. 4A illustrates data traffic transmitted in the downlink from basestation 102 to subscriber station 116 according to an exemplaryembodiment of the present disclosure. An exemplary frame of OFDM data is10 milliseconds in length and comprises fifteen (15) transmit timeintervals (TTIs), namely TTI 1 through TTI 15, where each one of TTI 1through TTI 15 is 0.667 milliseconds in duration. Within each of TTI 2through TTI 15, there are four OFDM data symbols, where each OFDM datasymbol is 0.1667 milliseconds in duration. In the first TTI, namely TTI1, there are three OFDM data symbols that are preceded by a pilotpreamble symbol. The pilot preamble symbol is used by SS 116 to performsynchronization channel estimation and to determine the OFDM symbolprocessing parameter set.

FIG. 4B is a flow diagram illustrating the determination of the userchannel type based on the measurements on the preamble according to anexemplary embodiment of the disclosure. In an OFDM system, a known pilotsequence is transmitted for one or more OFDM symbol durations. Channelestimation block 285 in the receiver (i.e., SS 116) detects the knownpilot signal, which is then use to perform synchronization (process step410). Channel estimation block 285 uses the detected preamble symbols todetermine the degree of multipath effects in the channel and, therefore,the frequency selectivity in the channel between BS 102 and SS 116(process step 420).

Based on the profile of the channel, channel estimation block 285 (oranother processing element or controller in SS 116) determines (i.e.,calculates) a set of OFDM symbol processing parameters (i.e., gainvalues and delay values) that may be used in BS 102 to improve receptionof OFDM symbols in SS 116 (process step 430). SS 116 then feeds back theOFDM symbol processing parameter set to BS 102 in the uplink (processstep 440). Other factors, such as mobile speed, can also be used indetermining (or calculating) the OFDM symbol processing parameters. Thechannel type may also be determined by using other mechanisms, such asreference in time-frequency.

In this manner, BS 102 receives an OFDM symbol processing parameter setfrom each subscriber station. Thereafter, as BS 102 schedules eachsubscriber station to receive data, BS 102 uses the OFDM symbolprocessing parameter set for that subscriber station to modify the OFDMsymbols transmitted from each antenna for BS 102. For example, BS 102may use OFDM Symbol Processing Parameter Set A to transmit OFDM symbolsfrom two or more antennas to SS 116 and may use OFDM Symbol ProcessingParameter Set B to simultaneously transmit OFDM symbols from two or moreantennas to SS 115.

FIG. 5 is a message flow diagram illustrating the transmission of OFDMsymbols from base station 102 to subscriber station 116 according to oneembodiment of the disclosure. In this example, base station 102 uses twotransmit antennas (first antenna ANT1 and second antenna ANT 2) totransmit to SS 116. SS 116 receives a first pilot signal (Pilot1) fromantenna ANT1 and receives a second pilot signal (Pilot2) from antennaANT 2. SS A then determines OFDM Symbol Processing Parameter Set A asdescribed above in FIGS. 4A and 4B.

Next, SS 116 transmits OFDM Symbol Processing Parameter Set A to BS 102in signal 505. Thereafter, BS 102 uses OFDM Symbol Processing ParameterSet A to transmit OFDM data symbols in the downlink back to SS 116. Asnoted above, the OFDM symbol processing parameters in Parameter Set Aconsist of symbol delays and gains from the two antennas. By way ofexample, in signal 520, BS 102 transmits from ANT1 processed OFDMsymbols that were processed using Parameter Set A. In signal 525, BS 102simultaneously transmits from ANT2 processed OFDM symbols that wereprocessed using Parameter Set A.

BS 102 also simultaneously transmits pilot signal 510 (Pilot1) and pilotsignal 515 (Pilot2) from the two transmit antennas, ANT 1 and ANT 2. Inthe embodiment in FIG. 5, Pilot1 and Pilot2 are not processed using theparameters in OFDM Symbol Processing Parameter Set A. This is due to thefact that another transmission may be scheduled at the same time foranother subscriber station on other OFDM subcarriers using a differentset of OFDM symbol processing parameters. The pilot signals must becorrectly understood by all the subscriber stations scheduled in thecell, so the pilot signals are not modified using OFDM Symbol ProcessingParameter Set A.

FIG. 6 is a flow diagram illustrating the processing of pilot signalsand OFDM data symbols according to an exemplary embodiment of thepresent disclosure. Because the OFDM symbols in signals 520 and 525 areprocessed using the values in OFDM Symbol Processing Parameter Set A,signals 520 and 525 are combined during transmission over the radio linkin such a way that single OFDM symbols are received in SS 116 from BS102 (process step 660). Since pilot signals 510 and 515 (Pilot1 andPilot2) are transmitted on orthogonal subcarriers from antenna ANT1 andantenna ANT2, pilot signals 510 and 515 are received separately at SS116 (process steps 605 and 610).

In order to get correct channel estimation for demodulation, SS 116compensates pilot signals 510 and 515 (Pilot1 and Pilot2) from antennasATN1 and ANT2 using the Parameter Set A received from channel estimationblock 285 (process steps 615 and 620). Compensated pilot signals 510 and515 are then combined (process step 630) and the overall channelestimate is obtained (process step 640). This overall channel estimateis then used by demodulator 280 to demodulate the processed data symbolscarried in the OFDM subcarriers (process step 660).

FIG. 7 is a message flow diagram illustrating the transmission of OFDMsymbols from base station 102 to subscriber station 116 according toanother embodiment of the disclosure. In FIG. 7, the OFDM symbolprocessing parameters are determined in base station (BS) 102, ratherthan in subscriber station (SS) 116. BS 102 may determine (or estimate)the OFDM symbol processing parameters in Parameter Set A from a numberof different uplink signals 705 transmitted by SS 116, including pilotsignals 705, preamble signals 705 and/or data signals 705 from SS 116.

In this example, since Pilot1 signal 710 and Pilot2 signal 715 are notprocessed using Parameter Set A, BS 101 transmits OFDM Symbol ProcessingParameter Set A to SS 116 in control message 720. SS 116 then uses theOFDM symbol processing parameters as described in FIGS. 2-6. BS 102transmits processed OFDM symbols 725 from ANT1 and processed symbols 720from ANT2 using the gain and delay values in Parameter Set A. SS 116uses the same gain and delay parameters in control message 720 tocompensate the pilots and to perform the overall channel estimation fordata demodulation.

FIG. 8 is a message flow diagram illustrating the transmission of OFDMsymbols from base station 102 to subscriber station 116 according toanother embodiment of the disclosure. Similar to FIG. 7, the OFDM symbolprocessing parameters in FIG. 8 are again determined in base station(BS) 102 for the case of two transmit antennas, rather than insubscriber station (SS) 116. BS 102 may determine (or estimate) the OFDMsymbol processing parameters in Parameter Set A from a number ofdifferent uplink signals 805 transmitted by SS 116, including pilotsignals 805, preamble signals 805 and/or data signals 805 from SS 116.

However, unlike FIG. 7, Pilot1 signal 810 from ANT1 and Pilot2 signal815 from ANT2 are processed using Parameter Set A. In this case, thePilot1 signal and the Pilot2 signal both use the same OFDM subcarriers.In other words, the two pilots are not transmitted on orthogonalsubcarriers. Therefore, the Pilot1 signal and the Pilot2 signal arereceived in SS 116 as a single signal that can be directly used foroverall channel estimation. The channel estimates are then used for datademodulation. BS 102 also transmits processed OFDM symbols 825 from ANT1and processed symbols 820 from ANT2 using the gain and delay values inParameter Set A.

In a scenario where the Pilot1 signal and the Pilot2 signal are notcompensated, the channel quality estimate is based on the pilot signalstrengths SS 116 receives from the two transmit antennas, ANT1 and ANT2.SS 116 compensates the Pilot1 signal and the Pilot2 signal using theOFDM symbol processing parameters. This gives an estimate of theexpected channel quality when BS 102 transmits OFDM symbols using theOFDM symbol processing parameters for SS 116. SS 116 then transmits achannel quality estimate (CQE) message back to BS 102. BS 102 determinesan optimum data rate based on the channel quality estimate (CQE) messagefrom SS 116 and then transmits processed OFDM symbols at that data rate.

In SS 116, processed OFDM symbols containing data are processed usinggain g0 from ANT1, gain g1 from ANT2 and delay D1 from ANT 2. Theseoperations reverse the operations in OFDM symbol processing block 230 inFIG. 3, assuming only transmit antenna 331 (i.e., ANT1) and transmitsantenna 332 (i.e., ANT2) are used. In SS 116, an FFT operation isperformed on the received OFDM symbols in order to retrieve theinformation in the frequency domain. The data and pilot symbols carriedon orthogonal subcarriers are separated in the frequency domain. Thepilot signals are converted back to the time domain by performing anIFFT operation. In this process, the subcarriers carrying data are setto 0. Also, when an ANT1 OFDM symbol is generated, the subcarrierscarrying ANT2 OFDM symbols are set to 0. Similarly, when an ANT2 OFDMsymbol is generated, the subcarriers carrying the ANT1 OFDM symbols areset to 0.

SS 116 multiplies the pilot OFDM symbols from ANT1 with gain g0 and thepilot OFDM symbols from ANT2 with gain g1. The receiver also delays thepilots from ANT2 with delay D1. Again, these operations reverse theoperations in OFDM symbol processing block 230 in FIG. 3, assuming onlytransmit antenna 331 (i.e., ANT1) and transmits antenna 332 (i.e., ANT2)are used. The two resulting pilots are then combined to get the overallpilot. An FFT operation is performed on the overall pilot to get theoverall channel response in the frequency domain. The channel estimatein the frequency domain is then used for data demodulation in thefrequency domain. This additional compensation on the pilot signalsallows for estimation of the additional processing done in BS 102 on theOFDM symbols containing data. The effect of the actual radio channel isalso reflected in the overall channel estimate because the receivedpilot signals travel via the radio channel.

The compensation needs to be done on the pilot symbols only, and not thedata symbols, because the data symbols are already processed in BS 102.In an OFDM system, the pilot and data symbols are carried on OFDMsubcarriers. Therefore, the compensation can either be done on the timedomain OFDM symbol or directly in the frequency domain. In order to docompensation in the frequency domain, the affect of OFDM symbol delay inthe time-domain must be accounted for in the frequency domain. Ingeneral, a time delay in the time domain translates into a phaserotation in the frequency domain. Therefore, the OFDM subcarrierscarrying the pilot symbols may be appropriately phase rotated in thefrequency domain to account for time delays.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The exemplary embodiments disclosedare to be considered as illustrative and not restrictive, and theintention is not to be limited to the details given herein. It isintended that the disclosure encompass all alternate forms within thescope of the appended claims along with their full scope of equivalents.

What is claimed is:
 1. For use in a subscriber station capable ofcommunicating with a base station of an orthogonal frequency divisionmultiplexing (OFDM) network, a method of controlling downlinktransmissions to the subscriber station, the method comprising:receiving a first pilot signal from a first antenna of the base station;receiving a second pilot signal from a second antenna of the basestation; estimating the channel between the base station and subscriberstation based on the received first and second pilot signals;determining a set of OFDM symbol processing parameters based on theestimated channel, wherein the OFDM symbol processing parameters areusable by the base station to control the relative gains and therelative delays of OFDM symbols transmitted from the first and secondantennas; and transmitting the OFDM symbol processing parameter set tothe base station, wherein the relative delays of the OFDM symbolstransmitted from the first and second antennas are cyclic delays.
 2. Themethod as set forth in claim 1, wherein determining the set of OFDMsymbol processing parameters is based on multipath characteristics andfrequency selectivity characteristics of the channel.
 3. The method asset forth in claim 2, further comprising: receiving first processed OFDMsymbols transmitted from the first antenna of the base station; andreceiving second processed OFDM symbols transmitted from the secondantenna of the base station, wherein the first and second processed OFDMsymbols are processed using the OFDM symbol processing parametersreceived from the subscriber station.
 4. The method as set forth inclaim 3, further comprising: demodulating the first and second processedOFDM symbols.
 5. The method as set forth in claim 3, further comprising:receiving a further first pilot signal from the first antenna; receivinga further second pilot signal from the second antenna; compensating thefurther first pilot signal according to the OFDM symbol processingparameters to generate a first compensated pilot signal; compensatingthe further second pilot signal according to the OFDM symbol processingparameters to generate a second compensated pilot signal; and combiningthe first and second compensated pilot signals to generate a combinedpilot signal.
 6. The method as set forth in claim 5, further comprising:further estimating the channel based on the combined pilot signal. 7.The method as set forth in claim 6, further comprising: demodulating thefirst and second processed OFDM symbols using the channel estimate basedon the combined pilot signal.
 8. A subscriber station capable ofcommunicating with a base station of an orthogonal frequency divisionmultiplexing (OFDM) network, wherein the subscriber station comprises:receive path circuitry configured to receive a first pilot signal from afirst antenna of the base station, and receive a second pilot signalfrom a second antenna of the base station; and channel estimatingcircuitry configured to estimate the channel between the base stationand subscriber station based on the received first and second pilotsignals, and determine a set of OFDM symbol processing parameters basedon a channel quality estimate, wherein the OFDM symbol processingparameters are usable by the base station to control the relative gainsand the relative delays of OFDM symbols transmitted from the first andsecond antennas, wherein the subscriber station is configured totransmit the OFDM symbol processing parameter set to the base station,and wherein the relative delays of the OFDM symbols transmitted from thefirst and second antennas are cyclic delays.
 9. The subscriber stationas set forth in claim 8, wherein the channel estimating circuitrydetermines the set of OFDM symbol processing parameters based onmultipath characteristics and frequency selectivity characteristics ofthe channel.
 10. The subscriber station as set forth in claim 9, whereinthe receive path circuitry is configured to receive first processed OFDMsymbols transmitted from the first antenna of the base station, and toreceive second processed OFDM symbols transmitted from the secondantenna of the base station, wherein the first and second OFDM symbolshave been processed by the base station using the OFDM symbol processingparameters transmitted by the subscriber station.
 11. The subscriberstation as set forth in claim 10, further comprising: demodulationcircuitry configured to demodulate the first and second processed OFDMsymbols.
 12. The subscriber station as set forth in claim 10, whereinthe receive path circuitry is configured to receive a further firstpilot signal from the first antenna and receive a further second pilotsignal from the second antenna and wherein the channel estimatingcircuitry is configured to: compensate the further first pilot signalaccording to the OFDM symbol processing parameters to generate a firstcompensated pilot signal, compensate the further second pilot signalaccording to the OFDM symbol processing parameters to generate a secondcompensated pilot signal, and combine the first and second compensatedpilot signals to generate a combined pilot signal.
 13. The subscriberstation as set forth in claim 12, wherein the channel estimatingcircuitry is configured to estimate the channel based on the combinedpilot signal.
 14. The subscriber station as set forth in claim 13,further comprising: demodulation circuitry configured to demodulate thefirst and second processed OFDM symbols using a channel estimate basedon the combined pilot signal.